1. Field of the Invention
The present invention relates to a radio transmission and reception system for transmitting and receiving digital data by radio, and in particular, to a digital radio transmission and reception system applying a direct modulation and demodulation method.
2. Description of the Related Art
In general, a digital radio transmission and reception system modulates digital data for transmission as an analog type to thereby transmit the modulated digital data by radio, as well as to receive the radio-transmitted signal to thereby demodulate the received signal into digital data. Here, for example, a frequency shift keying (hereinafter, referred as to FSK) modulation, a phase shift keying (hereinafter, referred as to PSK) modulation, and etc. are widely used to modulate the digital data. An example of a FSK modulator and demodulator is described in U.S. Pat. No. 5,311,556 to William E. Baker and entitled Digital FSK Transmitter Receiver And Method Of Operating Same, and examples of different FSK demodulators are described in U.S. Pat. No. 5,550,505 to Richard C. Gaus, Jr., and entitled Frequency Shift Keying (FSK) Demodulator For use In Low Signal-To-Noise Ratio Environments and U.S. Pat. No. 4,570,125 to Rodney W. Gibson and entitled FSK Demodulator With Concurrent Carrier And Clock Synchronization.
One use of a digital radio transmission and reception system is for a digital cordless telephone widely used in a mobile radio communication network or a private radio switching network. The aforesaid digital cordless telephone forms a radio terminal having functions for modulating the digital data into an analog signal to thereby transmit the modulated digital data (the analog signal), as well as to receive the analog signal by radio to thereby demodulate the received analog signal into the digital data. The aforesaid digital data includes speech signals as the analog signals belonging to an audible frequency band or tone signals for link setting. Representatively, an embodiment of the above digital cordless telephone can be a digital European cordless telephone.
The transmitting part and the receiving part of a digital European cordless telephone, however, includes a complicatedly constructed frequency processing circuit because it uses a single frequency conversion method or a dual frequency conversion method. Therefore, there is a problem with the digital European cordless telephone in that it is difficult to manufacture miniaturized and low-priced terminals required for cordless telephones. Besides, there is frequently generated phenomenon that frequency shift keying of the receiving signal is incapable of correction due to fading in an area where there are many obstacles, such as residential buildings. Therefore, during "open loop" conversion to transmit the data having a constantly equal logic level, for example, data "1111" or "0000", a problem may occur of momentary frequency skip of a voltage controlled oscillation signal generated when a difference in phase is set to "high impedance" through allowing a phase locked loop to be under a state of power down.